'International Standard' cartridges are a common pattern in use by most European and Japanese fountain pen manufacturers. Fountain pens designs will adopt one of two approaches for cartridges: proprietary designs that force you to buy that brand of cartridge, ensuring they receive the ongoing income from refills; or the standard design, which gives you a choice of cartridge brands to use in your pen.
Notable brands that don't accept standard cartridges are: Parker, Sheaffer, Cross, Platignum, Platinum and Lamy.
Major brands that DO use standard cartridges are Faber-Castell, Kaweco, rotring, Porsche, Caran d'Ache, Waterman, Tombow, OHTO, Pelikan, Visconti and Schneider. The advantage of buying a fountain pen from one of these manufacturers is that you have an enormous choice of cartridges available to you with many more colour options, and in an emergency you should be able to find a basic refill in any stationery shop.
There are two versions of the international standard cartridge - 'short' 38mm cartridges and 'long' 73mm cartridges. Most pens will accept either, and many are designed to accommodate two short cartridges - one in use and one spare in the barrel. This should ensure that you don't run out of ink unexpectedly. A few very compact fountain pens can only use the short version.
All our standard cartridges are of excellent quality, though Diamine Ink should be noted as offering particularly high quality along with the widest colour range. We like their ink so much that we've had some cartridges made especially for us, in our own Cult Pens Deep Dark colours!